RoboteQ AX2550/2850 user manual Using the Encoder to Measure Speed

Using the Encoder to Measure Speed

Using the Encoder to Measure Speed

The encoder module will automatically compute rotation speed for each encoder. The resulting measured speed is a value ranging from 0 to + 127 and 0 to -127, where 127 rep- resent a relative ratio of a maximum speed value chosen by the user.

For example, if the encoder module is configured so that the highest measured speed value is 3,000 RPM, then a reading of 63 (127/2) would be 1,500 RPM.

The relationship between the measured speed and the actual speed is a factor of two vari- able parameters: a Time Based period value stored inside the Encoder module and the Encoder’s number of Pulses per Revolution. Note: the Encoder’s number of Pulses per Revolution is not stored in the controller.

The Time Base is a number of 256us time intervals between two counter reads.

A simple procedure is included in the Roborun PC utility to easily determining and set these parameters.

For information, the exact formula is shown below:

Measured Speed Value = RPM * PPR * 4 * (Time Base+1) * 256 / (60 * 1000000) or Measured Speed Value = RPM * PPR * (Time Base + 1) / 58593.75

Example: a motor spinning at 1,000 RPM, with an encoder with 200 Pulses per Revolution, and a Time Base set at 4 will produce the following measurement:

1000 *200 * (4 + 1) / 58593.75 = 17

The same formula modified to show the actual RPM at a given Measure Speed Value is as follows:

RPM = Measured Speed Value * 60 * 1000000 / (PPR * 4 * 256 * (Time Base+1)) or RPM = Measured Speed Value * 58593.75 /((Time Base + 1) * PPR)

In our example, a measured speed value of 127 correspond to the following measurable max actual RPM values

RPM at Max Measurable Speed Value = 127 * 58593.75 / ((4 + 1) * 200) = 7441 RPM

A measured speed value of 1 correspond to the following measurable min. actual RPM val- ues

RPM at Min. Measurable Speed Value = 1 * 58593.75 / ((4 + 1) * 200) = 58.6 RPM

The Roborun Utility automatically makes the above calculations when setting up the encoder.

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